WASHINGTON — Representative Peter T. King, the longest-serving Republican in New York’s congressional delegation, said on Monday that he would retire, joining a growing exodus of Republicans from Congress ahead of the 2020 elections.
Mr. King, who is currently serving his 14th term in the House, said in a statement that his decision was primarily motivated by the desire to end the weekly commute to Washington after nearly three decades and have “more flexibility” to spend time with his children and grandchildren.
But in an interview, Mr. King conceded that the toxic political environment in Washington and the uncertainties of a coming impeachment proceeding against President Trump had underscored his sense that it was time to leave.
“Something was building for a while,” Mr. King said. The difficulties of coordinating his Thanksgiving and Christmas travel plans with family amid the unknowns of the impeachment battle, he said, “somehow became like a metaphor” fueling his decision.
The exit of Mr. King, 75, comes as a growing number of Republicans have decided to retire rather than seek re-election as they eye the grim political realities for their party, including an uphill slog to win back the House in next year’s election and the prospect of sharing a ticket with an unpopular president. The dilemma is particularly acute for Republicans in suburban districts, where voters have been alienated by Mr. Trump’s rhetoric and his positions. Mr. King is the 20th House Republican to depart Congress at the end of 2020 because of retirement or seeking another elected office.
Mr. King, who represents parts of Nassau County and Suffolk County as part of New York’s second congressional district, said none of those concerns influenced his decision, pointing to his favorable polling numbers and a sizable war chest. More persuasive, he said, was his daughter’s recent move to North Carolina, and a desire to give New York Republicans plenty of time to find a replacement candidate ahead of primary in April.
“I think it’s gotten too toxic,” Mr. King, a boxer, acknowledged, “but once I’m in the arena, I enjoy the battle.”
He said he called Mr. Trump on Sunday to inform him of his plans, assuring the president that he would use his final months in office to vote against articles of impeachment and support Mr. Trump’s re-election bid, as Republicans seek to prevent any defections in the coming weeks. The president asked him to reconsider retirement, Mr. King said, but ultimately accepted his decision.
“We can’t allow impeachment to become a political issue, like a tax bill or a highway bill,” Mr. King said, recalling his vote against impeachment articles against President Bill Clinton. “My concern right now is that it’s become part of the political landscape now.”
“When the word ‘impeachment’ was used, there was a quiver, a hesitation — just using the word just shook people up,” he added. “And now there’s none of that.”
Mr. King is also the latest in a series of seasoned moderate lawmakers to decide to depart the House, a trend that will leave the next Congress with less expertise and experience, and with fewer members who can remember a time when they reached across party lines to enact bipartisan legislation. He singled out other members of the New York delegation — Representatives Elise Stefanik and John Katko, both Republicans, and Tom Suozzi, a Democrat — as members he would hope would carry on that bipartisan work. (Mr. Suozzi, for his part, said “Pete King is a friend” who “can always be relied on to say exactly what he thinks.”)
Mr. King built a reputation as one of the more moderate members of the Republican conference, but frequently took a hard line on immigration issues. As chairman of the House Homeland Security Committee, Mr. King oversaw congressional hearings on homegrown Islamic terrorism, which critics condemned as a forum for attacking American Muslims after the Sept. 11 attacks.
On Monday, some Muslim groups, including Muslim Advocates, a national civil rights organization, greeted the news of Mr. King’s departure with relief.
“Good riddance,” Representative Ilhan Omar, Democrat of Minnesota and one of the first Muslim women elected to Congress, wrote on Twitter.
Mr. King himself was once an ardent backer of a terrorist group, the Irish Republican Army, previously telling The New York Times that he had no regret for his statements of support. He has celebrated his past with novels that feature an Irish-American congressman with I.R.A. ties — a striking resemblance to Mr. King himself.
Senator Chuck Schumer of New York, the minority leader, praised Mr. King effusively on Monday upon the news of his retirement, saying that the congressman “stood head & shoulders above everyone else.”
“He’s been principled & never let others push him away from his principles,” Mr. Schumer wrote on Twitter, quickly drawing public rebukes from Democrats. “He’s fiercely loved America, Long Island, and his Irish heritage and left a lasting mark on all 3.”
Mr. King is the third New York lawmaker to announce his departure at the end of the 116th Congress, following two veteran Democrats, Representative José E. Serrano, the nation’s longest-tenured Hispanic congressman, and Representative Nita M. Lowey, the chairwoman of the House Appropriations Committee. (Representative Chris Collins, the fourth-term Republican who pleaded guilty to federal securities fraud charges, already departed the chamber earlier this year.)
Representative Cheri Bustos of Illinois, who heads the House Democratic campaign arm, said that Mr. King’s district “has been a pickup target of ours from day one of this cycle, and we will compete to win it in 2020.”
“Republicans know their toxic health care repeal agenda and wholesale embrace of President Trump’s recklessness will guarantee they remain in the minority for years to come,” she said in a statement. Liuba Grechen Shirley, who lost to Mr. King last year by six points, said on Monday she was “seriously considering another run” for the seat.
Chris Pack, a spokesman for the Republican counterpart, countered in a statement thanking Mr. King for his service, saying that the district “will remain a Republican seat, thanks in no small part to the insane socialist agenda of Democrats in both Washington and Albany.”
In his statement announcing his departure, Mr. King promoted his work after the Sept. 11 attacks, his efforts to lead the storm recovery after Hurricane Sandy and his work with Mr. Clinton to negotiate the Good Friday peace agreement in Northern Ireland. Just this year, Mr. King also noted, Congress finally approved legislation that would care permanently for those who have grown deathly ill from the toxins of the rubble at the World Trade Center in Manhattan.
Mr. King, who also reflected on his childhood in Sunnyside and his work as a college student at Manhattan’s West Side Railway Terminal, vowed to remain politically active after his retirement — just not in the halls of Congress.
“I’m sure the next time something really dramatic happens, I’m going to wait for reporters to call and to go down to Washington,” Mr. King said. “But nobody will call, and I will have nowhere to go.”
“That’s what I’m going to miss,” he added, “being in the arena and being able to get things done, instead of just talking about it.”
Vivian Wang contributed reporting from New York.